The present invention relates generally to integrated circuits and more specifically to the use of bonded substrates to form integrated circuits capable of handling fiber optic elements and forming pressure transducers.
There is a substantial amount of research under way to increase the optical communication across an integrated circuit and to and from an integrated circuit. This is to replace the metal interconnects in an effort to reduce surface space needed as well as to minimize cross-talk. A typical example is shown in U.S. Pat. No. 4,549,338 to Abend et al. Optical fibers are positioned in and through a top of the package for the integrated circuit to provide optical transmission to and from areas on the integrated surface substrate. It has even been suggested to laminate the integrated circuit to a transparent insulative layer having a plurality of bores on the backside thereof to receive the fiber optic elements. This is shown in Japanese patent 55-151337 dated Dec. 25, 1980, to Ihara.
However, the general problem with prior art devices is the alignment of the fiber optical elements to the appropriate portion of the integrated circuit. Alignment of package lid to photo-element is complicated if a hermetic package is required. Maintaining alignment during package lid sealing (either brazed or glass seal) will be difficult. If a ceramic lid is used, sizing differences between the array of holes in the lid and the array of photo-elements may limit the overall number of photo-elements that can be integrated in this fashion.
Commercial pressure transducers on integrated circuits have generally included devices utilizing a single crystal silicon diaphragm onto which diffused resistors are patterned. Diaphragm size is on the order of 0.250".times.0.250".times.0.001", so only about 75 transducers can be fabricated on a 4" silicon wafer. Furthermore, formation of the diaphragm is usually accomplished by back etching the silicon wafer, so that the etched wafer must be attached to a glass (pyrex) substrate to form the reference cavity. This results in a structure involving at least five materials, causing a significant differential thermal expansion influence on transducer sensitivity.
An alternate method of fabricating the diaphragm is to use a sacrificial layer which is removed via lateral etching as described in "Fabrication Techniques for Integrated Sensor Macrostructures," by H. Guckel and D. W. Burns, 1986 IEEE IEDM, pp 176-179, or "Scaling Limits in Batch-Fabricated Silicon Pressure Sensors," by Hin-Leung Chau and Kensall D. Wise, IEEE, Electrical Device Transactions, Vol. ED-34, No. 4, 1987, pp. 850-858. These types of devices utilize a diaphragm composed of a deposited film such as polysilicon. Because the thickness of this deposited film can be well-controlled and the overall diaphragm thickness is in the range of a few microns, diaphragm size can be reduced resulting in a higher number of transducers fabricated on a silicon wafer.
One final example of state-of-the-art pressure transducer fabrication can be seen in "Micro-Diaphragm Pressure Sensor," by S. Sugiyama et al., 1986 IEEE IEDM, pp. 184-187. Here, a Si.sub.3 N.sub.4 layer is used for the diaphragm and the reference chamber is etched using a combination of a lateral etch of a polysilicon interlayer and an anisotropic etching of the silicon substrate forming a pyramid shaped cavity. Although diaphragm size is reduced to 80.mu..times.80.mu., an alkali etchant (KOH) is used, which may contaminate oxides which are already present on the wafer surface. This is of special concern if MOS interface circuitry is placed alongside the transducer.
Thus it is an object of the present invention to provide a new structure for receiving and aligning fiber optic elements to photoelectric interface circuits.
Another object of the present invention is to provide an economic and reliable method of manufacturing integrated circuits including fiber optic elements and photoelectric interface circuits.
An even further object of the present invention is to provide an improved pressure transducer in an integrated circuit.
A still even further object of the present invention is to provide a pressure transducer in an integrated circuit requiring a minimum amount of area and greater diaphragm thickness control.
These and other objects are attained by forming moats in a first substrate having a bottom and at least one slanted side wall. A second substrate extends across the moat and includes a photoelectric interface circuit. A fiber optic element extends axially along the bottom wall terminating adjacent the slanted side wall of the moat and is in optical communication with the photoelectric interface circuit by the slanting side wall which has a reflective surface. The reflective surface is an oxide and has a thickness to produce reflective characteristics. The two substrates are bonded together by an oxide and any bonding oxide which is in the optical path between the photoelectric interface device and the fiber optics element has a thickness which permits light transmission. The photoelectric device may be, for example, a diode which can be a photo receiver or transmitter.
The method of forming the integrated circuit with the fiber optic element includes forming at least one moat on a first surface of the first substrate, which is part of a wafer, forming a reflective surface on the slanted wall and bonding a second wafer to the first wafer, and forming a photoelectric interface device in the second wafer above the slanted wall and dividing the wafer at the scribe line to form at least one die with the moat exposed at its side wall opposite the slanted wall. The fiber optic element is then inserted through the open wall to extend along the bottom wall and terminate adjacent the slanted wall. The moat with the slanted wall is formed by anisotropic etching and the reflective surface may be formed as an oxide layer. The oxide reflective layer is formed during the processing forming the photoelectric interface device. The second wafer has a thickness reduced after bonding and before device formation. Bonding is accomplished by providing a substantially thick bonding layer on one of the two surfaces and a very thin bonding layer on the other. The combined structure is then heated to produce a bond. The thickness of the post-bond bonding oxide layer is selected to permit light transmission.
A modification of the above process can be used to form a pressure transducer. The pressure transducer includes a cavity in the first surface of a substrate covered by a covering layer to seal the cavity. A sensor is formed on the cover layer above the cavity of a material which changes its electric characteristics as a function of the differential pressure above the sensor and in said cavity. A typical sensor could be a thin film resistor acting as a strain gauge which changes its electrical characteristics as it is bent. The method of fabrication includes forming a cavity in the first surface of the first substrate and applying a second substrate to the first substrate with a bonding layer on the second substrate. The combined structure is heated to bond the first and second substrates to each other. The second substrate is then completely or partially removed leaving the bonding layer sealing the cavity. The pressure sensor is formed on the bonding layer or in the thin silicon layer of the second substrate over the cavity.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.